The invention relates to a method for use in a cellular radiotelephone system to protect the data base of a visitor location register against saturation.
The Special Mobile Group (GSM) of the European Conference of Post and Telecommunications Administrations has laid down standards for implementing a European digital cellular radiotelephone system. This system includes a radio subsystem constituted by: mobile stations providing mobile subscribers with access to telecommunications services; base stations; and base station controllers. The system also includes another subsystem constituted by: mobile service switching centers; home location registers; visitor location registers; and operation and maintenance centers.
A base station provides radio transmission and control of the low layers of the signalling channels. A base station controller controls one or more base stations and acts as a bridge between base stations and a mobile service switching center. It manages radio resources and it performs preprocessing that enables the mobile service switching center to be made less specific to the "mobile radiotelephone" application, compared with a conventional telephone switching center.
A mobile service switching center enables the cellular radiotelephone system to interwork with fixed telecommunications networks, either directly or else via an interworking unit. Radio coverage of a territory is obtained by juxtaposing zones comprising one or more cells, each cell being served by a base station. A mobile service switching center handles the calls of mobile subscribers situated in one or more zones. These functions include certain features specific to the mobile nature of the subscribers, and in particular managing the automatic handover of calls from one cell to another. It also has another major difference compared with an exchange in a fixed telecommunications network: it does not have its own data concerning the mobile subscribers present in its location zones nor does it perform the associated management.
This management is performed by an entity called the visitor location register (VLR), with data concerning these mobiles being handed over from a central entity referred to as the home location register (HLR). A moving mobile subscriber passes successively from one location zone to another. Subscriber displacement is signalled automatically by the moving equipment to the base stations which inform the VLR supervising the corresponding location zones.
The term "in-booking" is used herein to designate the procedure whereby a VLR recovers data from the HLR concerning a mobile subscriber unknown thereto, with this procedure marking the arrival of the mobile subscriber in a location zone supervised by the VLR. The term "out-booking" is used herein to designate the procedure whereby a VLR releases the resources that it had previously allocated to a subscriber when the subscriber leaves the zones belonging to the VLR, and after this event has been notified by the HLR. The term "updating the internal location" is used herein to designate the procedure whereby a VLR updates information relating to the location zone of a mobile known thereto, and which marks a subscriber moving from one of the zones supervised by the VLR to another.
The in-booking procedure consists essentially in: authenticating the mobile and obtaining data from the HLR describing the subscription of the mobile. The HLR has two main functions: it constitutes the reference data base for the cellular radiotelephone system; and it keeps track continuously of the location of the mobile subscribers to enable incoming calls to be directed to them. In a national network there may be one or more HLRs. In a cellular radiotelephone system, mobile subscribers are identified by: a directory number enabling them to be called from another subscriber in the fixed network or from another mobile subscriber, and by a permanent international mobile station identity (IMSI) which is allocated to the equipment of the mobile station and which is used on the fixed network and possibly also on the radio segment.
In order to be able to guarantee that the international identity remains confidential, the VLR allocates a temporary mobile station identity (TMSI) to each subscriber, with the TMSI being used as often as possible on the radio segment instead of the IMSI. The GSM specifications define the way in which the various identities of the mobile subscriber should be used during various procedures.
The data base of an HLR contains the description of all of the mobile subscribers under its control. This data base is managed by a real time data management system (SGDTR) which provides quick access to data. The data in the data base is organized in the form of two-dimensional tables. Each row of a table (also known as an n-tuplet) is allocated to a different subscriber, and each column contains a particular type of information describing subscribers, for example information concerning a service to which they may subscribe. The intersection of a row and a column thus corresponds to a particular item of information relating to one subscriber.
For each mobile subscriber, the data base of an HLR contains:
the national directory number; PA1 the list of services subscribed-to; PA1 the international identity or IMSI; PA1 location, i.e. the identity of the VLR supervising the zone in which the subscriber is present; and PA1 data enabling the mobile to be authenticated, which data is referred to below as authentication triplets. PA1 the national directory number; PA1 the list of services subscribed to; PA1 the international identity IMSI; PA1 the temporary identity, TMSI; PA1 the location zone; and PA1 the authentication triplets. PA1 as soon as a mobile is booked into a VLR, the VLR allocates it an n-tuplet in its data base; and PA1 this data is of use only if the mobile requires attention either by moving or else by making a call. PA1 allocating a temporary mobile identity word to each mobile that announces its presence for the first time in a geographical zone served by the register, said word comprising a first field specifying an internal reference for an n-tuplet in a table of the data base, and a second field which is a function of the instant at which the temporary identity word is allocated; PA1 writing each temporary identity word in the n-tuplet designated by its first field; PA1 releasing said n-tuplet when the mobile demonstrates its presence in a zone corresponding to another register; PA1 allocating a new temporary identity word to a mobile each time said mobile again announces its presence in a zone served by the register after a silence of duration greater than a threshold value, with the first field thereof remaining unchanged while the second field thereof being a function of the instant at which the mobile again announces its presence; PA1 writing the new temporary identity word in the n-tuplet designated by the first field; and PA1 periodically releasing from the data base those n-tuplets which correspond to temporary identity words whose second fields correspond to instants earlier than a first threshold instant; PA1 the method being characterized in that it further consists, whenever the data base is filled close to saturation, in releasing n-tuplets from the data base corresponding to temporary identity words whose second fields correspond to instants earlier than a second threshold instant which is more recent than the first threshold instant.
The data base of an HLR is updated: whenever a subscription is opened, altered, or cancelled by the network operator; whenever a subscriber alters one of the services subscribed to, e.g. a call transfer number; whenever a subscriber changes VLR; and whenever new authentication triplets need to be calculated. The SGDTR serves to define access keys for each table. The IMSI and the directory number are the access keys to subscriber data. When an n-tuplet is allocated, the SGDTR provides the number of this n-tuplet which is an internal reference for the data base enabling the data to be accessed more quickly than when using an access key such as the IMSI.
Each VLR also has its own data base controlled by the same real time data control system SGDTR. This data base contains data describing mobile subscribers currently located in any one of the location zones supervised by the VLR, and mainly constituted by:
The subscriber number, the international identity IMSI, and the temporary identity TMSI are access keys to the subscriber data in the VLR data base.
Changes in the VLR data base are very frequent since it is altered each time there is in-booking, out-booking, an internal location update, a subscription data update concerning mobile subscribers in the VLR, or a renewal of authentication triplets. The in-booking procedure requires the SGDTR to allocate n-tuplets in the various tables storing information describing subscribers, and the out-booking procedures requires all of these n-tuplets to be released. The data describing the mobile is received either from the VLR which was supervising it previously (authentication triplets), or else from the HLR (all data). The in-booking procedure includes informing the HLR of the new location of the mobile.
The SGDTR manages tables of fixed size only. It is therefore not possible to allocate a number of n-tuplets which is greater than the size of the table. For the HLR, this does not constitute a constraint since the number of subscribers is under the control of the network operator. However, for a VLR which manages subscribers that are mobile by definition, it is necessary to be able to take account of the following constraints:
Patent application EP-A-0 260 763 describes a method of managing the data base of a visitor location register by storing words in said data base in association with each mobile subscriber to identify one or more geographical zones in which the subscriber has recently required attention. Each zone-identifying word is accompanied by another word specifying the time at which the mobile station required attention in said zone. The probability of the subscriber being present in each of these zones is estimated by comparing the times that have elapsed since the most recent calls concerning the subscriber that have taken place in respective ones of the zones. A word identifying a zone in association with a subscriber is deleted from the data base of the visitor location register whenever said subscriber has neither made nor received a call in said zone over a predetermined length of time. This length of time is made longer whenever only one zone-identifying word remains stored in the data base. A subscriber no longer present in any of the zones served by the register under consideration thus ceases to occupy space in said data base once a certain length of time has elapsed. There nevertheless remains a risk that the data base will saturate.
At peak times, it may happen that the number of mobile subscribers present in the location zones of the VLR exceeds the capacity of the VLR. In this case, the VLR can either: refuse to book new mobiles in; or else it can unilaterally book out mobiles that have not required attention for a certain length of time.
It is always possible for the VLR to retrieve data from the HLR concerning a subscriber that it has been out-booked, but only at the cost of a procedure which is expensive in computation time, and that is to be avoided when operating at full load.
A VLR may suffer partial or total breakdown, which leads to the need to be able to re-establish subscriber data as and when subscribers require attention by means of a procedure referred to as "restoring" the VLR.